Dynamic changes of Au/ZnO catalysts during methanol synthesis: A model study by temporal analysis of products (TAP) and Zn LIII near Edge X-Ray absorption spectroscopy

Aligning time-resolved kinetics (TAP) and surface spectroscopy (AP-XPS) for a more comprehensive understanding of ALD-derived 2D and 3D model catalysts

Spectro-kinetic characterization of complex catalytic materials, i.e. relating the observed reaction kinetics to spectroscopic descriptors of the catalyst state, presents a fundamental challenge with a potentially significant impact on various...

Fundamental Aspects of Ceria Supported Au Catalysts Probed by In Situ/Operando Spectroscopy and TAP Reactor Studies

The discovery of the activity of dispersed gold nanoparticles three decades ago paved the way for a new era in catalysis. The unusual behavior of these catalysts sparked many questions about their working mechanism. In particular, Au/CeO₂ proved to be an efficient catalyst in several reactions such as CO oxidation, water gas shift, and CO₂ reduction. Here, by employing findings from operando X‐ray absorption spectroscopy at the near and extended Au and Ce L_III energy edges, we focus on the fundamental aspects of highly active Au/CeO₂ catalysts, mainly in the CO oxidation for understanding their complex structure‐reactivity relationship. These results were combined with findings from in situ diffuse reflectance FTIR and Raman spectroscopy, highlighting the changes of adlayer and ceria defects. For a comprehensive understanding, the spectroscopic findings will be supplemented by results of the dynamics of O₂ activation obtained from Temporal Analysis of Products (TAP). Merging these results illuminates the complex relationship among the oxidation state, size of the Au nanoparticles, the redox properties of CeO₂ support, and the dynamics of O₂ activation.

Understanding Solid-Gas Reaction Mechanisms by Operando Soft X-Ray Absorption Spectroscopy at Ambient Pressure

Ambient-pressure operando soft X-ray absorption spectroscopy (soft-XAS) was applied to study the reactivity of hydroxylated SnO₂ nanoparticles toward reducing gases. H₂ was first used as a test case, showing that the gas phase and surface states can be simultaneously probed: Soft-XAS at the O K-edge gains sensitivity toward the gas phase, while at the Sn M_4,5-edges, tin surface states are explicitly probed. Results obtained by flowing hydrocarbons (CH₄ and CH₃CHCH₂) unequivocally show that these gases react with surface hydroxyl groups to produce water without producing carbon oxides and release electrons that localize on Sn to eventually form SnO. The partially reduced SnO_2 - x layer at the surface of SnO₂ is readily reoxidized to SnO₂ by treating the sample with O₂ at mild temperatures (>200 °C), revealing the nature of "electron sponge" of tin oxide. The experiments, combined with DFT calculations, allowed devising of a mechanism for dissociative hydrocarbon adsorption on SnO₂, involving direct reduction of Sn sites at the surface via cleavage of C-H bonds and the formation of methoxy- and/or methyl-tin species at the surface.

Stabilization by Configurational Entropy of the Cu(II) Active Site during CO Oxidation on Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O

The mechanisms of CO oxidation on the Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O high-entropy oxide were studied by means of operando soft X-ray absorption spectroscopy. We found that Cu is the active metal and that Cu(II) can be rapidly reduced to Cu(I) by CO when the temperature is higher than 130 °C. Co and Ni do not have any role in this respect. The Cu(II) oxidation state can be easily but slowly recovered by treatment of the sample with O₂ at ca. 250 °C. However, it should be noted that CuO is readily and irreversibly reduced to Cu(I) when it is treated with CO at T > 100 °C. Thus, the main conclusion of this work is that the high configurational entropy of Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O stabilizes the rock-salt structure and permits the oxidation/reduction of Cu to be reversible, thus permitting the catalytic cycle to take place.

Influence of CO on the Activation, O-Vacancy Formation, and Performance of Au/ZnO Catalysts in CO2 Hydrogenation to Methanol

The impact of CO on the activation and reaction characteristics of Au/ZnO catalysts in methanol synthesis from a CO₂/H₂ mixture was studied by kinetic, near ambient pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy at the O K-edge, together with in situ Foureir transform infrared measurements. Transient measurements under up to industrial reaction conditions (50 bar, 240 °C) show a pronounced transient increase of the activity for methanol formation from CO₂/H₂ after exposure to a CO/H₂ reaction gas mixture, while the steady-state activity is similar to that observed directly after oxidative pretreatment. For the reaction in CO/H₂, the much longer activation phase is accompanied by formation of CO₂ due to reaction of CO with the ZnO catalyst support. This leads to O-vacancy formation on the support at an extent significantly higher than in CO₂/H₂. The consequences of these findings on the mechanistic understanding of methanol formation from CO₂/H₂ on Au/ZnO and for ZnO-supported catalysts in general are discussed.